The quality of an image or a product formed on a substrate by ink jetted from an ink jet printer can be affected by the performance of jets in the printhead of the printer. The jets in some printheads are arranged in one or more rows, in a direction different from, e.g., perpendicular to, a process direction of the printer. Each jet includes a pumping chamber to receive and pump ink and a nozzle to jet ink from the pumping chamber to the substrate. By applying an activation voltages to a piezoelectric element associated with each pumping chamber ink droplets can be jetted based on information about the image to be printed.
Typically, the jets in each row are identical and each pair of neighboring jets along a row are separated by equal spaces. Each row of jets can be about 1 inch to about 3 inches long and can contain at least 25 jets or 50 jets and up to about 500 jets, for example. Each jetted ink droplet can have a size of about 2 picoliters to about 100 picoliters, based on dimensions of the jet and the voltages applied to the jet.
Generally, a jet is built for jetting one size of ink droplet in response to a particular activation voltage at a jetting frequency that is within a particular range. If the voltage varies or the jet is activated at a frequency outside the frequency range, the jet may perform poorly or even stop working. Sometimes a jet is built for jetting several different-sized ink droplets, each in response to a particular activation voltage and within a certain frequency range of jetting. Discussion of different types of printheads and jets is provided, for example, in U.S. Pat. No. 5,265,315, U.S. Pat. No. 7,052,117, U.S. Ser. No. 10/800,467, filed Mar. 15, 2004, U.S. Ser. No. 11/652,325, filed Jan. 11, 2007, and U.S. Ser. No. 12/125,648, filed May 22, 2008, all of which are incorporated here by reference.
Even when a jet is driven at the intended activation voltage and within the intended frequency range, the quality of the ink droplets (and the resulting printing) can be degraded by manufacturing flaws in, or a temporary malfunction of, the jet (air bubbles, or ink adhering to the nozzle, for example). Temporary malfunctions sometimes can be corrected.
The performance of a jet can be gauged in several ways. One technique analyzes quantifiable properties of ink droplets that it jets, for example, their size, speed, or trajectory. Another approach compares its performance to the performance of other jets in the row, for example, the response of the jet upon activation relative to the other jets or the speed of the jetted ink droplets relative to ink droplets jetted by the other jets. The performance can also be gauged by analyzing an image or product the jet prints, for example, information about whether a dot printed by the jet appears at an intended position with an intended size and shape on the substrate or whether a line printed by the jet is straight and has an intended thickness.
As shown in FIGS. 1A and 1B, in step-and-repeat printing, a printer 10 having one or more printheads 12 (not all shown) each containing one or more rows of jets 14 (not all shown) prints lines 16 on a substrate 18 that is stationary. The printhead 12 scans across a width of the substrate 18 along a rail 20 (process direction y) and prints lines 22 of successive dots that are parallel to the row of jets 14 (x direction). In this example, each line 22 corresponds to one jet 14 in the row of jets and the density of the lines 22 along the x direction depends on the density of jets 14 in the row. The substrate 18 then moves a step along the x direction and the printhead 12 repeats the printing process across the substrate 18.
Referring to FIG. 1B, in single pass printing, a stationary printer 24 having one or more printheads 34 (not all shown) each containing one or more rows of jets 28 (not all shown) covers a width of an image that is intended to be printed on a substrate 26 (x direction) and prints lines 30 continuously. The printer 24 prints successive rows of dots 32 parallel to the row of jets (x direction) when the substrate 26 passes under the jets 28 along the process direction y.